Bitter taste is imparted by a variety of compounds of widely varying structure. Large individual differences in the perception of these compounds have been well documented. The best known demonstration of individual differences in bitter taste sensitivity is seen with PROP and PTC. However, how strongly one perceives a particular concentration of PROP is not a reliable predictor of how strongly one perceives particular concentrations of many other bitter substances. PROP sensitivity has been shown to be related to fungiform papilla density and to the number of taste buds per papilla. The overall goals of this proposal are 1) to determine how several bitter tasting compounds cluster together as a function of individuals' differential sensitivities to the compounds' bitter tastes, and 2) to assess whether the correlation between anterior tongue taste buds and PROP sensitivity is causal. To this end, the specific aims of this proposal are to a) identify perceptual clusters of bitter compounds by assessing individual differences in bitter taste perception via ranking and paired-comparison procedures, b) investigate regional differences in sensitivity within the oral cavity for a variety of bitter compounds, allowing for the further grouping of compounds by location of sensitivity and possibly implicating variance in the relative densities of different bitter- sensitive receptors throughout the oral cavity, and c) determine if the stimulation of equivalent numbers of taste buds (independent of fungiform papilla density) or stimulation of equivalent sized areas of tongue results in similar perceived intensity ratings for PROP and other bitter compounds.